Chronic periodontitis affects a large fraction of the population. Although microbes are clearly the etiologic agents, no organism or group of organisms has been unequivocally identified as the primary cause of disease. Molecular exploration of several other natural microbial ecosystems has demonstrated a surprising variety of uncultivated organisms. Microscopy studies of oral flora have demonstrated the association of uncultivated spirochetes with several forms of periodontitis, and recent molecular studies have demonstrated their association with RPP. It is likely that pathogenic oral species such as spirochetes or other, less distinctive morphotypes remain undetected. Studies are proposed to determine if uncharacterized or previously unsuspected bacterial species are present in subjects with periodontitis using a molecular approach that circumvents the need for culturing. The bacterial rDNA genes from plaque samples will be amplified with universal primers and cloned. After prescreening to remove common, characterized species, novel clones will be identified by sequencing. Novel or previously unsuspected species will be examined for their association with disease by examining a bank of existing bacterial DNA samples collected from subjects with periodontitis and periodontally healthy controls. The presence and levels of these species will be determined by quantitative, real-time PCR. The same methods will be used to examine the association of 6 previously implicated cultivable species with periodontitis. Comparisons among species will be made. Periodontitis appears to be, at least to some degree, transmissible between close contacts. Consistent with this, the putative periodontal pathogens P. gingivalis and A. actinomycetemcomitans also appear to be frequently transmitted between close family contacts. Children often harbor these organisms, particularly when parents are colonized. In children the colonization appears to be transient. In contrast, in adults with periodontitis these bacteria appear to be extremely difficult to eradicate. Studies are proposed to examine the acquisition, stability and family transmission of the 4 most strongly disease-associated species. The presence and quantity of each species will be determined by quantitative, real-time PCR.